The process according to the present invention relates to the treatment of waste water discharged from a bleach plant and more particularly treatment of bleach plant alkaline wash filtrate from treatment-steps and/or delignification/bleaching-steps of pulp.
When trying to close up the water circuits of lignocellulose bleach plants, in order to reduce the environmental impact of that bleaching, there is a growing need for a process with which it is possible to recycle a portion of the bleach plant""s washer filtrates to the brownstock washing fiberline and thereby reduce the ultimate discharge of pollutants to the environment. Especially important is the development of a process which will reduce the discharge of AOX (absorbable organic halogen), BOD (biological oxygen demand), COD (chemical oxygen demand), colors and other potentially regulated parameters.
Generation of large amounts of bleach plant waste water is of course disadvantageous and contradictory to the aim of closing up the bleach plant. If too much bleach plant washer filtrates are recycled in the pulp mill, the impact on the chemical recovery system will be too high. Hence, process equipment such as the black liquor evaporators are not constructed for recycle of large amounts of waste water. Additionally, process equipment in the recovery system is very sensitive to the introduction of chlorides and other inorganic substances from the recovery of bleaching wash filtrates. Increased concentrations of inorganic cations and anions resulting from bleach plant closure may cause many negative effects including the formation of scale deposits (incrustations), corrosion and consumption of bleaching chemicals.
Basta et al. xe2x80x9cPartial Closure in Modem Bleaching Sequences,xe2x80x9d Proceedings of TAPPI International Pulp Bleaching Conf. (1996), Tappi, pp 341-346, teaches the importance of keeping elementary chlorine free (ECF) alkaline effluents separate from acid effluents. The acid effluent is more responsive to biological waste treatment and contains a higher level of elements that are undesirable to return to the pulping liquor cycle.
Full counter-current washing is a well known operation for decreasing the amount of fresh wash water used in washing and dewatering the pulp. However, it is not desirable to mix alkaline and acidic streams in the bleach plant, which occurs when strictly counter-current washing is applied. In bleaching operations, for example in the case of elemental chlorine free (ECF) bleaching, it is desirable not to concentrate the inorganic substances along with the organic substances. Acid waste water and alkaline waste water from a bleach plant comprise different amounts of inorganic and organic material and have varying salt contents. Thus, the alkaline waste water comprises a great deal of relatively high molecular weight organic material and sodium alkali, whereas the acid waste water primarily comprises lower molecular weight organics and inorganic salts, such as calcium, magnesium, chloride, chlorate etc. If the acid and the alkaline waste water are mixed, it becomes more difficult to separate the different substances from the process flow in a subsequent treatment, which is disadvantageous in methods aiming at closing up the bleach plant. As little as possible of the chlorides should be concentrated with the organics. Mixing of alkaline and acidic streams may result in problems due to the formation of reaction products, so-called incrustations, which are difficult to separate. The incrustations also clog up washing equipment, heat exchange equipment and evaporators, thus necessitating frequent stoppages for chemical and mechanical cleaning of the unit operations. Foam formation may also occur when treating mixed waste water which can result in operating difficulties or increased costs for defoaming agents.
Blackwell et al, xe2x80x9cRecycle of Bleach Plant Extraction Stage Effluent to the Kraft Liquor Cycle: A Theoretical Analysisxe2x80x9d, Int. Chemical Recovery Conf. (1992), Tappi, pp 329-350, discloses a process for recycling bleach plant extraction stage filtrate without pre-treatment for reduction of AOX, BOD, color, toxicity etc. The conclusion was that up to 50% of the extraction stage filtrate effluent could be directly recycled into the brownstock fiberline at a point with weight % dissolved solids similar to those contained in the alkaline filtrate, but with a 16% increase in chemical recovery evaporation load.
Afonso et al, xe2x80x9cTreatment of Bleaching Effluents by Pressure-driven Membrane Processesxe2x80x94A Reviewxe2x80x9d, in xe2x80x9cMembrane Technology: Applications to Industrial Waste Water Treatment, Kluwer Academic Publishers (1995), pp. 63-79, summarize prior art documents which use membrane filtration of bleach plant filtrates to concentrate organics into a volumes of less than xe2x85x9 of the initial volume with the intent to bypass the brownstock fiberline and take the concentrate directly back to the chemical recovery system for evaporation prior to combustion.
Blackwell et al, xe2x80x9cUltrafiltration of Kraft Bleach Plant Effluent: Process Design and Cost Estimatexe2x80x9d, Int. Environmental Conf. (1992), Tappi, pp 603-614, discloses a process for recycling bleach plant extraction stage filtrate with pretreatment by ultrafiltration for reduction of AOX, BOD, color, toxicity etc. The conclusion was that most of the extraction stage filtrate effluent could be treated by ultrafiltration to reduce its volume to approximately {fraction (1/15)} of its initial volume, then recycle it directly into the weak black liquor to achieve an overall bleach plant reduction of 40% color and 25% AOX with a 7% increase in chemical recovery evaporation load. Additionally, this prior art teaches that any recycle of alkaline filtrate concentrate into the brownstock fiberline must be done at a point were the weight % dissolved solids equal or exceed those in the alkaline concentrate.
Known processes have shown the need for, and the problems associated with, the recovery of alkaline filtrates into the chemical recovery system. Improvements for solving the problems have been shown, but further improvement is needed to: 1) increase total recovery efficiency of COD, AOX and other dissolved high molecular weight organics; 2) reduce additional evaporation load in chemical recovery; 3) reduce restrictions on where the alkaline concentrate can be introduced into the fiberline.
From the known processes, it is obvious that strictly counter-current washing of bleach plant wash filtrates to the brownstock washing fiberline should be avoided, if aiming at reduction of hazardous compounds for the environment. A direct recycle of primarily alkaline wash filtrate from the bleach plant is preferred. For example, in the use of a so called jump-stage washing configuration, a bleach plant""s alkaline stage filtrate may be taken around its preceding bleach stage which has material undesirable to recycle, for example an acid stage filtrate, to any preceding brownstock washer while sewering the jumped bleach stage filtrate. In a direct recycling of bleach plant alkaline filtrate for use as wash or dilution water on the pulp system, e.g. with the use of the above mentioned jumpstage washing, it is also important to use relatively precise amounts of wash water. In such a washing operation, the amount of dissolved solids in the washing that can be pushed back to the chemical recovery cycle is limited by washer efficiencies and by the hydraulic balance. The key problem with using jump stage washing of bleach plant alkaline filtrate as shower water for the last brownstock pulp washer is that most of the alkaline filtrate applied in excess of the brownstock fiberline washer dilution factor (OF), will carry forward most of that excess alkaline filtrate to the jumped bleach stage , and exit with the jumped stage""s filtrate. In general, this hydraulic constraint will limit jump stage partial bleach plant closure to 20-50% of the total bleach plant alkaline filtrate flow. In the case where the dissolved organics not are concentrated into a volume small enough, in the recirculation of alkaline filtrates for introduction upstream of an open bleach stage, the carryover of dissolved organics into the open bleach stage would be great (open bleach stage means sewering of the bleach filtrate, e.g. for external treatment or mill receiving water body). In addition, a large volume of bleach plant alkaline wash filtrate used in place of any brownstock wash or dilution water other than fresh water, which is usually only applied to any significant degree to the last brownstock washer, would imply a significant increase on the evaporation load in chemical recovery. These problems or drawbacks can be eliminated to a large extent when using the technique according to the invention.
Thus, when using bleach plant alkaline filtrate as wash water for the brownstock fiberline it is important that the recycled alkaline filtrate comprises a reduced amount of low molecular weight inorganics other than sodium and sulphur, while the largest possible amount of organic substances are enriched in the concentrated alkaline filtrate for recycle. Treatment of the alkaline filtrates before recirculation should ideally reduce the low molecular weight inorganics such as Cl31 ,CO3xe2x88x922, C2O4xe2x88x922, SO4xe2x88x922 ,Ba+2, Mg+2, and Ca+2 in approximately the same proportion to the water volume reduction, making this stream more desirable for return to the brownstock washing area. Therefore, the inorganics should preferably follow equally with the water split, if not enriched in the non-organic stream.
The organics in the class of resin acids and sterols are considered potentially toxic, and a high retention of these in the concentrate is achievable. To maximize their retention, the volume reduction is kept within an optimal range, lower than recommended by known processes. If the volume reduction in the concentrate of the alkaline filtrates is too high, less of these organics will remain in the concentrate stream.
According to the present invention a process is provided for reducing the amount of dissolved organic compounds in waste water discharged from a bleach plant, as described in the claims. In the process, alkaline wash filtrate from treatment-steps and/or delignification/bleaching-steps of pulp is treated in a process by the following steps: i) treating at least a part of the alkaline filtrate in order to produce an alkaline concentrate enriched in high molecular weight dissolved organic compounds and a stream depleted in organic compounds, where the share of the alkaline concentrate to the total amount of the alkaline filtrate corresponds to the volume reduction factor (VRF), which is in the range of from about 2.0 up to about 7.5; ii) withdrawing the depleted stream; iii) collecting at least a part of the alkaline concentrate from step i) and using the concentrate as wash or dilution water in brownstock washing fiberline with a dilution factor of up to about 4.0, where at least a part of said washing filtrate is further brought in one or more steps to the chemical recovery system.